The present invention relates to a mechanically operable suction device for lifting and holding objects, particularly air-permeable objects.
In the art of handling objects, there exist a number of applications in which gripping elements, lifting magnets or the like cannot be employed for lifting, holding and moving objects. In such cases, resort frequently is made to vacuum-operated suction devices. Such devices, however, may present problems when the objects to be handled are at least partly permeable to air. If such objects cannot be lifted by devices other than suction devices, then the suction devices and the associated vacuum systems must be designed for air flows of considerable magnitude to ensure the creation of effective pressure differentials capable of generating forces of sufficient strength for lifting and holding the particular objects.
One example of such an application is the functional testing of pre-drilled, but as yet unassembled printed circuit boards by means of automatic circuit board testing machines. During such operation, it is desirable to lift and move without difficulty printed circuit boards consisting of a non-magnetic material so that the handling of such boards may be automated as much as possible. It of course is not possible to lift non-magnetic circuit boards by means of magnetic forces, and the use of mechanical gripping devices is not desirable since the circuit board contours and dimensions may change frequently. Thus, the logical alternative is to employ a vacuum-operated suction apparatus, but again this type of solution presents problems due to the fact that pre-drilled circuit boards are permeable to air.
Because of the substantial air flows required, it is obvious not to activate such a suction device until the pressure differential it generates is needed, i.e. at the time the device engages the object to be lifted. Also, it is desirable that the mechanisms required for activating the suction device do not unnecessarily increase the air flows which are needed in the first place.
A suction device including a plunger for mechanically activating the same is disclosed in DE-A1-No. 31 01 864, and the present invention represents an improvement of such device. In accordance with such device, a spring is used to bias a valve member toward a valve port so as to seal the port in a non-active condition of the device. In the active condition of the suction device, the spring contributes to reducing the lifting forces of the device and/or to increasing the suction air requirements of the device because the return force of the spring increases with the opening of the intake port.